The present invention relates to a ball screw used as a feed screw for use with machine tools, presses, steering gears for automobiles, carrying devices, elevating devices and so forth.
Generally, in machine tools or the like, it is necessary to transform rotary motion into linear motion. Such transformation needs to be carried out efficiently and accurately.
Conventionally, a ball screw accomplishes this. Japanese Patent Laid-Open No. 242866/1989 discloses a ball screw having screw shaft with spiral grooves formed in its outer circumference. The screw shaft is rotatably retained by a bearing relative to a rotating main body. This bearing includes balls, a retainer for retaining the balls at certain intervals, and an outer ring for guiding the balls. The retainer is not secured to any of the outer ring, rotating main body, or screw shaft.
Where there is no slippage between the outer ring and the balls, one rotation of the screw shaft accurately advances the screw shaft by a distance equal to one pitch of the groove thereof. However, where a great magnitude of force is imparted to the balls of the bearing by abruptly stopping the rotation of the screw shaft, slippage occurs between the outer ring and the balls. This causes the balls to move along the grooves of the screw shaft, while the screw shaft axially moves such that its rotation relative to the rotating main body stops. When this occurs, it is not possible to accurately transform rotary motion into linear motion, or linear motion into rotary motion.